Search

• Extensive new Ba isotopic survey of modern barites. • First Proterozoic Ba isotopic results. • Provides genetic model for enigmatic post-Marinoan barite formation. • Provides evidence for stratified post-Marinoan ocean. • Provides arguments for why barite horizons are confined to Marinoan and not Sturtian or Paleoproterozoic strata. Measurements of triple oxygen isotope ratios in barite horizons within post-Marinoan cap carbonates have provided some of the most compelling evidence that the Marinoan glaciation was a Snowball Earth event. However, the origin of these barite horizons remains unresolved. To constrain the Ba sources, and thus formation mechanisms of these horizons, we analyzed the Ba isotope composition of post-Marinoan barite deposits from Northwest Canada, Northern Norway, Brazil and South China. We augment these analyses with a Ba isotope survey of almost 100 modern and ancient additional barite measurements, including samples from pelagic (or 'marine'), hydrothermal, terrestrial, Proterozoic stratiform and cold seep environments. Unlike modern cold seep or terrestrial barites, we find that globally-distributed post-Marinoan barites exhibit a relatively narrow isotopic range, suggesting a well-mixed, effectively limitless Ba source. Moreover, post-Marinoan deposits exhibit a similar mean Ba isotope composition to modern marine barites, which we interpret as evidence of a marine Ba source. Considered alongside existing geochemical, geological, and new Ba isotope survey data, we conclude that Ba in barite horizons was sourced from a well-mixed, Ba-replete but SO 4 -poor reservoir that accumulated during the Marinoan Snowball Earth interval. This deep Ba reservoir was then transported upward—either by ocean circulation or dolomitization of underlying cap carbonates—and was brought into contact with continental weathering-derived sulfate in a post-glacial meltwater surface layer. Thus, in addition to providing a plausible mechanism for generating globally-synchronous deposition of post-Marinoan barite horizons that reconciles all existing geochemical and geological data, our results demonstrate the utility of Ba isotopes to interrogate the origin of enigmatic barite deposits throughout the sedimentary record. [ABSTRACT FROM AUTHOR]

We present a new set of high precision measurements of relative 33 S/ 32 S, 34 S/ 32 S, and 36 S/ 32 S values in lunar mare basalts. The measurements are referenced to the Vienna-Canyon Diablo Troilite (V-CDT) scale, on which the international reference material, IAEA-S-1, is characterized by δ 33 S = −0.061‰, δ 34 S ≡ −0.3‰ and δ 36 S = −1.27‰. The present dataset confirms that lunar mare basalts are characterized by a remarkable degree of sulfur isotopic homogeneity, with most new and published SF 6 -based sulfur isotope measurements consistent with a single mass-dependent mean isotopic composition of δ 34 S = 0.58 ± 0.05‰, Δ 33 S = 0.008 ± 0.006‰, and Δ 36 S = 0.2 ± 0.2‰, relative to V-CDT, where the uncertainties are quoted as 99% confidence intervals on the mean. This homogeneity allows identification of a single sample (12022, 281) with an apparent 33 S enrichment, possibly reflecting cosmic-ray-induced spallation reactions. It also reveals that some mare basalts have slightly lower δ 34 S values than the population mean, which is consistent with sulfur loss from a reduced basaltic melt prior to eruption at the lunar surface. Both the sulfur isotope homogeneity of the lunar mare basalts and the predicted sensitivity of sulfur isotopes to vaporization-driven fractionation suggest that less than ≈1–10% of lunar sulfur was lost after a potential moon-forming impact event. [ABSTRACT FROM AUTHOR]

Abstract: Sulfur isotope studies of post-Archean terrestrial materials have focused on the ratio 34S/32S because additional isotopes, 33S and 36S, were thought to carry little information beyond the well-known mass-dependent relationship among multiple-isotope ratios. We report high-precision analyses of Δ33S and Δ36S values, defined as deviations of 33S and 36S from ideal mass-dependent relationships, for international reference materials and sedimentary sulfides of Phanerozoic age by using a fluorination technique with a dual-inlet isotope ratio mass spectrometer. Measured variations in Δ33S and Δ36S are explained as resulting from processes involve branching reactions (two or more reservoirs formed) or mixing. Irreversible processes in closed systems (Rayleigh distillation) amplify the isotope effect. We outline how this new isotope proxy can be used to gain new insights into fundamental aspects of the sulfur biogeochemical cycle, including additional constraints on seawater sulfate budget and processes in sedimentary sulfide formation. The isotope systematics discussed here cannot explain the much larger variation of Δ33S and Δ36S observed in Archean rock records. Furthermore, Phanerozoic samples we have studied show a characteristic Δ33S and Δ36S relationship that differs from those measured in Archean rocks and laboratory photolysis experiments. Thus, high precision analysis of Δ33S and Δ36S can be used to distinguish small non-zero Δ33S and Δ36S produced by mass-dependent processes from those produced by mass-independent processes in Archean rocks and extraterrestrial materials. [Copyright &y& Elsevier]

Abstract: The discovery of mass-independent isotope effects observed in Archean rocks, certain classes of meteorites, and atmospheric aerosols has had profound implications to our understanding of ancient and present atmospheric sulfur chemistry. We present a new technique that takes advantage of continuous He flow isotope-ratio-monitoring gas chromatography–mass spectrometry to achieve precise analysis of all four stable sulfur isotopes (32S, 33S, 34S, and 36S) at nanomole level samples. The technique involves fluorination of sulfide (silver sulfide or pyrite), and separation of product gas by gas chromatography and the removal of mass-131 interference by a liquid-nitrogen ethanol slush at −110 °C. This technique works with an optimum sample size of 100 to 200 nmol with precision for Δ33S and Δ36S at 0.1 and 0.5‰ (2σ). Samples, as small as tens of nanomole, can be analyzed using this new method. One of the major sources of error in irm-GCMS is found to be tailing of the major ion beam (32SF5 +) onto minor beams (33SF5 + and 36SF5 +), which results in contraction of the measured δ 33S and δ 36S scales. This effect is corrected by measuring a series of reference sulfide samples with mass-dependent sulfur isotope compositions. This methodology increases the spatial resolution of the laser ablation in situ analysis and considerably reduces the analysis time as compared with conventional dual inlet methods. [Copyright &y& Elsevier]

• Computational modeling of BMCs enlightens experimentally hard-to-measure features. • Significant exploration remains of catabolic BMC capabilities. • Oxygen sensitivity is relatively common across BMC-contained enzymes. • BMC shell proteins are selectively permeable to negatively charged metabolites. Bacterial microcompartments (BMCs) are protein-encapsulated compartments found across at least 23 bacterial phyla. BMCs contain a variety of metabolic processes that share the commonality of toxic or volatile intermediates, oxygen-sensitive enzymes and cofactors, or increased substrate concentration for magnified reaction rates. These compartmentalized reactions have been computationally modeled to explore the encapsulated dynamics, ask evolutionary-based questions, and develop a more systematic understanding required for the engineering of novel BMCs. Many crucial aspects of these systems remain unknown or unmeasured, such as substrate permeabilities across the protein shell, feasibility of pH gradients, and transport rates of associated substrates into the cell. This review explores existing BMC models, dominated in the literature by cyanobacterial carboxysomes, and highlights potentially important areas for exploration. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents worked solutions for the fractionations of all four stable sulfur isotopes (32S, 33S, 34S, and 36S) in several models of the sulfate reduction metabolism. We describe methods for obtaining solutions and how the predictions made by these solutions define different compositional fields (phase space) that can be used to gain new insights into sulfur metabolisms, specifically with respect to understanding the structure of and fractionations associated with the network of reactions that describe the transformations of sulfur within the cell. We show how this treatment can be used to evaluate data from experiments with dissimilatory sulfate reducers and to suggest that the expression of fractionations by the metabolic process is largely limited by the fraction of sulfate that is lost from the cell, and that the variation in observed fractionations reflects differences in the proportion of sulfur intermediates that are reoxidized to sulfate. This analysis provides a line of support for this assertion that depends only on the sulfur isotopic fractionations between sulfate and sulfide. This analysis also indicates that internal fractionations are consistent with a relationship given by 33 α =(34 α) θ where α is the fractionation factor (e.g., 33 α a−b =(33 S/32S)a/(33S/32S)b and 34 α a−b =(34 S/32S)a/(34S/32S)b), and where θ is restricted to a value between 0.515 and 0.514. This finding is consistent with a control on isotopic fractionation effects within the cell that is rooted in the different partition coefficients (energetics) for the different isotopologs. [Copyright &y& Elsevier]

• CAS preserves fast metabolic rates of AOM-SR. • Pyrite integrates evolving sulfide under variable methane fluxes and oxidative cycling. • Protracted carbonate precipitation under non-steady state methane seepage. The anaerobic oxidation of methane coupled with sulfate reduction (AOM-SR) is a major microbially-mediated methane consuming process in marine sediments including methane seeps. The AOM-SR can lead to the formation of methane-derived authigenic carbonates which entrap sulfide minerals (pyrite) and carbonate-associated sulfate (CAS). We studied the sulfur isotope compositions of the pyrite and CAS in seafloor methane-derived authigenic carbonate crust samples from the North Sea and Barents Sea which reflect the time-integrated metabolic activity of the AOM-SR community as well as the physical conditions under which those carbonates are formed. In these samples, pyrite exhibits δ 34 S values ranging from − 23.4 ‰ to 14.8‰ and Δ 33 S values between −0.06‰ and 0.16‰, whereas CAS is characterized by δ 34 S values ranging from 26.2‰ to 61.6‰ and Δ 33 S mostly between −0.05‰ and 0.07‰. Such CAS sulfur isotope compositions are distinctly lower in δ 34 S- Δ 33 S space from published porewater sulfate values from environments where the reduction of sulfate is mostly coupled to sedimentary organic matter oxidation. Mass-balance modelling suggests that (1) AOM-SR appears to cause rapid carbonate precipitation under high methane flux near or at the sediment-water interface and (2) that the precipitation of pyrite and carbonates are not necessarily synchronous. The sulfur isotopic composition of pyrite is interpreted to reflect more variable precipitating conditions of evolving sulfide with porewater connectivity, fluctuating methane fluxes and oxidative sulfur cycle. Taken together, the multiple isotopic compositions of pyrite and sulfate in methane-derived authigenic carbonates indicate protracted precipitation under conditions of non-steady state methane seepage activity. [ABSTRACT FROM AUTHOR]

Iron isotopes have been widely applied to investigate the redox evolution of Earth’s surface environments. However, it is still unclear whether iron cycling in the water column or during diagenesis represents the major control on the iron isotope composition of sediments and sedimentary rocks. Interpretation of isotopic data in terms of oceanic redox conditions is only possible if water column processes dominate the isotopic composition, whereas redox interpretations are less straightforward if diagenetic iron cycling controls the isotopic composition. In the latter scenario, iron isotope data is more directly related to microbial processes such as dissimilatory iron reduction. Here we present bulk rock iron isotope data from late Proterozoic marine shales from Svalbard, northwestern Canada, and Siberia, to better understand the controls on iron isotope fractionation in late Proterozoic marine environments. Bulk shales span a δ 56 Fe range from −0.45 ‰ to +1.04 ‰ . Although δ 56 Fe values show significant variation within individual stratigraphic units, their mean value is closer to that of bulk crust and hydrothermal iron in samples post-dating the ca. 717–660 Ma Sturtian glaciation compared to older samples. After correcting for the highly reactive iron content in our samples based on iron speciation data, more than 90% of the calculated δ 56 Fe compositions of highly reactive iron falls in the range from ca. −0.8 ‰ to +3 ‰ . An isotope mass-balance model indicates that diagenetic iron cycling can only change the isotopic composition of highly reactive iron by < 1 ‰ , suggesting that water column processes, namely the degree of oxidation of the ferrous seawater iron reservoir, control the isotopic composition of highly reactive iron. Considering a long-term decrease in the isotopic composition of the iron source to the dissolved seawater Fe(II) reservoir to be unlikely, we offer two possible explanations for the Neoproterozoic δ 56 Fe trend. First, a decreasing supply of Fe(II) to the ferrous seawater iron reservoir could have caused the reservoir to decrease in size, allowing a higher degree of partial oxidation, irrespective of increasing environmental oxygen levels. Alternatively, increasing oxygen levels would have led to a higher proportion of Fe(II) being oxidized, without decreasing the initial size of the ferrous seawater iron pool. We consider the latter explanation as the most likely. According to this hypothesis, the δ 56 Fe record reflects the redox evolution of Earth’s surface environments. δ 56 Fe values in pre-Sturtian samples significantly heavier than bulk crust and hydrothermal iron imply partial oxidation of a ferrous seawater iron reservoir. In contrast, mean δ 56 Fe values closer to that of hydrothermal iron in post-Sturtian shales reflects oxidation of a larger proportion of the ferrous seawater iron reservoir, and by inference, higher environmental oxygen levels. Nevertheless, significant iron isotopic variation in post-Sturtian shales suggest redox heterogeneity and possibly a dominantly anoxic deep ocean, consistent with results from recent studies using iron speciation and redox sensitive trace metals. However, the interpretation of generally increasing environmental oxygen levels after the Sturtian glaciation highlights the need to better understand the sensitivity of different redox proxies to incremental changes in oxygen levels to enable us to reconcile results from different paleoredox proxies. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes a comparison of ultraviolet photolysis experiments undertaken with SO2 (oxygen with isotopes at natural abundance levels) and S18O2 (18O-substituted oxygen). Experiments were conducted in a closed photocell using a deuterium lamp (principally 190–235nm) under pressure regimes (5–25Torr) that produced optically thick conditions for 32SO2 and variable optical depths for other isotopologues. The experiments, which were designed to examine the effects of intramolecular isotopic substitution of oxygen atoms on the S-MIF produced during UV photolysis of SO2, reveal generally reduced sulfur fractionation for 18O-rich SO2 as compared to 16O-rich SO2. Model shielding calculations were undertaken using spectra that were shifted due to changes in rotational and vibrational energy levels. The model calculations suggest that processes in addition to rotational and vibrational shifts in absorption spectra play a role in the experimentally produced isotope effects. Such additional processes may include differences in primary photoexcitation arising from smaller peak-to-valley amplitudes for fine structure of 18O-rich SO2 absorption spectra or an isotopically selective process associated with transitions between excited states. [Copyright &y& Elsevier]

Dissolved sulfate is a crucial electron acceptor for the subsurface biosphere, particularly for the living microbial ecosystems in the long-isolated (on the order of millions to billions of years) deep subsurface fracture waters in Precambrian cratons, e.g., in the Witwatersrand Basin of the Kaapvaal Craton, South Africa. Aiming to understand the role of sulfate in the sustainability of the subsurface habitat and the spatial extent of the terrestrial subsurface biosphere, we carried out a basin-scale examination on the sources and producing mechanisms of dissolved sulfur (sulfate and sulfide) in the subsurface fracture waters in the Witwatersrand Basin using multiple sulfur isotopes (32,33,34,36S) and oxygen isotopes (16,18O). Dissolved sulfates in 14 fracture water samples collected from 900 to 3,413 meters below the surface at 6 sites show isotopic ranges from −5.3‰ to +19.4‰ for δ 34S, −0.40‰ to +0.50‰ for Δ33S, and −1.1‰ to +10.9‰ for δ 18O. These isotopic signatures are distinct to those of the sulfate minerals (e.g., the carbonate-associated sulfate in local Transvaal Supergroup dolomite sequences: δ 34S = +31.4‰ to +39.2‰; Δ33S = −0.01‰ to +0.16‰), but identical to those of the sulfide minerals in the host rocks. This indicates that the dissolved sulfate in the fracture waters were dominantly generated by in situ oxidation of sulfide minerals at basin scale, although mixing of a small amount of surface sulfate at some sites cannot be completely ruled out. The dissolved sulfates in the less deep and less saline fracture waters are in oxygen isotope disequilibrium with their host waters, a surprising result given that the water residence times are orders of magnitude longer than the time required for oxygen isotope exchange to reach equilibrium. This implies that vigorous in situ sulfate production has occurred after the fracture waters were isolated. In contrast, the dissolved sulfate in the deeper, more saline waters are in apparent oxygen isotope equilibration or close to equilibration with their host waters. This might be attributed to a combined effect of faster oxygen isotope exchange between sulfate and water at higher temperatures, larger extent of sulfate reduction, and/or less efficient sulfate production. The dissolved sulfide in the fracture waters has similar Δ33S values but is 3.0‰ to 26.4‰ lower in δ 34S than coexisting sulfate, suggesting that the dissolved sulfide is mostly generated from bacterial sulfate reduction, which is consistent with the widespread existence of sulfate-reducing bacteria down to >3.4 km below surface in the Witwatersrand Basin. Overall, these novel isotopic results demonstrate that geological processes can provide a steady long-term sulfate source in deep fracture fluids by in situ oxidation of sulfide minerals in their host rocks, and thereby a mechanism to sustain the terrestrial subsurface biosphere, even in deep, high-temperature, long-isolated water systems. Thus, sulfate as a terminal electron acceptor is not the limiting factor for the spatial expansion of terrestrial subsurface biosphere. • Dissolved sulfates in subsurface fracture waters across the basin show large S-MIF. • Coexisting dissolved sulfide has similar S-MIF but much lower δ 34S values. • The S-MIF are not consistent with host-rock sulfate minerals but sulfide minerals. • Dissolved sulfate is formed by long-standing in situ oxidation of sulfide minerals. • The in situ sulfate production facilitates expansion of the biosphere to great depth. [ABSTRACT FROM AUTHOR]

Abstract: Here, we present new measurements of 32S, 33S, 34S, and 36S in sedimentary sulfides and couple these measurements with modeling treatments to study the sulfur cycle of a late Paleoproterozoic marine basin. We target the transition in ocean chemistry from the deposition of Paleoproterozoic iron formations (Gunflint Formation, Biwabik Formation, Trommald Formation, and Mahnomen iron formations) to the inferred sulfidic ocean conditions recorded by overlying shale (Rove Formation). The data suggest that certain features of the global sulfur cycle, such as a control by sulfate reducing prokaryotes, and low (mM) concentrations of oceanic sulfate, were maintained across this transition. This suggests that the transition was associated with changes in the structure of the basin-scale sulfur cycle during deposition of these sediments. Sulfide data from the iron formations are interpreted to reflect sedimentary sulfides formed from microbial reduction of pore-water sulfate that was supplied through steady-state exchange with an overlying oceanic sulfate reservoir. The sulfide data for the euxinic Rove Formation shales reflect the operation of a sulfur cycle that included the loss of sulfide by a Rayleigh-like process. We suggest that the prevalence of large and variable heavy isotope enrichments observed in Rove Formation sulfide minerals reflect a sustained and significant net loss of sulfide from the euxinic water column, either as a result of a shallow chemocline and degassing to the atmosphere or as a result of a water column pyrite sink. The inclusion of 36S measurements (in addition to 32S, 33S, and 34S) illustrates the mass-dependent character of these sedimentary environments, ruling out contributions from the weathering of Archean sulfides and pointing to at least modest levels of sustained atmospheric oxygen (>10−5 present atmospheric levels of O2). [Copyright &y& Elsevier]